1. Field of the Invention
The present invention generally relates to transmission power correcting methods, mobile communications system, and mobile stations, and, more particularly, to a transmission power correcting method, a mobile communications system and a mobile station in which a transmission power of a source of transmission is corrected, based on a reception quality measured at a destination of transmission.
2. Description of the Related Art
For downstream transmission in a mobile communications system, i.e. data transmission from a base station to a mobile station, a technology known as high speed downlink packet access (HSDPA) is used for high-speed, large-volume downloading. A discussion on HSPDA underway in 3rd Generation Partnership Project (3GPP) is directed to changing of a modulation scheme and a Turbo coding rate in accordance with the quality of reception at a mobile station. In changing a modulation scheme and a Turbo coding rate, it is necessary for a base station to notify a mobile station of the modulation scheme and the Turbo coding rate to be used. Particularly, in adaptive modulation coding (AMC) in which a modulation scheme and a Turbo coding rate are adaptively changed, transmission (signaling) of information related to the modulation coding scheme from the base station to the mobile station occurs frequently. 3GPP Technical Report (TR) 25.858V1.0.0 “8 Associated Signaling” (hereinafter, referred to as reference 1) gives a description of a signaling procedure related to this signaling. Information related to the modulation coding scheme includes transport-format and resource combination (TFRC).
FIG. 10 shows an example of TFRC list given in reference 1. The list is provided in a portion of reference 1 where uplink signaling is described. The list lists substantially the same information related to the modulation coding scheme transmitted from the base station to the mobile station in downlink signaling. The list lists combinations of a modulation scheme, a transport block set (TBS) size and the number of codes. The modulation scheme may be one of two digital modulation schemes including quaternary phase shift keying and (QPSK) and 16 quadrature amplitude modulation (QAM). A TBS size indicates the data size of a transport block (TrBlk) included in a frame multiplied by the number of blocks. That is, the TBS size indicates the data size of a frame. A TBS size is a parameter related to Turbo coding and is one type of information related to modulation coding scheme. It is assumed here that a multicode scheme, in which a plurality of channelization codes (spreading codes) are assigned to a mobile station, is used. The list lists the number of codes included in a multicode (in the illustration the number of codes is 5).
For example, TFRC1 includes parameters such that modulation scheme=QPSK, TBS size=1200 bit and number of codes=5. In the case of TFRC6, modulation scheme=16 QAM, TBS size=7200 bit and number of codes=5. Assuming that spreading factor (SF)=16 and one frame=2 ms, the volume of data per frame is 4800 bits when modulation scheme=QPSK, 9600 bits when modulation scheme=16 QAM. The Turbo coding rate is ¼ for TFRC(1), ½ for TFRC(2), ¾ for TFRC(3), ½ for TFRC(4), ⅝ for TFRC(5) and ¾ for TFRC(6). The information given above is not immediately available from the table of FIG. 10, though.
In transmitting the information (TFRC, according to reference 1) related to the modulation coding scheme from the base station to the mobile station, instead of transmitting the information as it is, identification data having a smaller data volume is transmitted. Identification data corresponds to transport-format and resource related information (TFRI) of reference 1. The data volume of TFRI is defined in reference 1 as follows.    Channelization code set: 7 bits    Modulation scheme: 1 bit    Transport block set size: 6 bits
A channelization code set indicates a combination of a plurality of channelization codes assigned to a mobile station according to a multicode scheme. FIG. 10, listing TFRCs, would not be complete without listing channelization code sets instead of only the number of codes. FIG. 10, however, serves the purpose since it corresponds to a special case where the number of codes is fixed to 5. Therefore, only the number of codes is given.
Traffic between the base station and the mobile station is reduced by employing an information transmission scheme in which the information related to the modulation coding scheme is converted into the identification data.
The mobile station of the mobile communications system measures the quality of reception. By feeding back the result of measurement to the base station, the transmission power of the base station is corrected to an appropriate level. More specifically, uplink signaling from the mobile station is used to inform the base station of a power offset value, based on the quality of reception measured by the mobile station. In accordance with the information obtained through signaling, the base station corrects the transmission power.
FIG. 11 shows a table of reference 1 listing power offset values. The table of FIG. 11 lists a plurality of power offset values for each TFRC listed in FIG. 10. An identification code is associated with each set of a TFRC and the power offset value. Power offset=0 dB is used as a default value in each set of a TFRC and a power offset value. Referring to FIG. 11, power offset values of 1 dB and 2 dB are prescribed for TFRC(2)-TFRC(6) other than the default power offset value of 0 dB. For TFRC(1), in addition to 0 dB, the power offset values up to 12 dB in steps of 1 dB are provided. By transmitting a desired power offset shown in FIG. 11 to the base station, based on the quality of reception measured by the mobile station, the power of transmission from the base station is subject to fine control so that the throughput of the entire system is improved.
However, in a system configuration as described in reference 1, where the mobile station only notifies the base station of an increase in the transmission power, the transmission power requested may exceed the total power, i.e. power rating, of the base station. Another disadvantage with the related-art system configuration is that, performing only an increase in the transmission power of the base station may induce an adverse effect of intra-cell interference or inter-cell interference.